In certain textile applications, high frequency noise sources require acoustical materials having good absorption characteristics. It is known, for example, that texturing air jets typically produce noise concentrated above 4000 Hz and predominantly in the 8000 Hz to 16,000 Hz octave bands.
Because textile machinery oftentimes operates in an atmosphere laden with dust and other particulate matter such as lint, acoustical material, such as 1 inch thick foam, must include a facing which is easily cleanable and which prevents contamination of the acoustical material. However, known facing material, such as fiberglass, coarse polyester fabrics and films such as vinyl and polyester, adversely affect the high frequency noise absorption characteristics of the acoustical material.
It is the dual object of this invention to provide a facing material which does not impair good high frequency absorption characteristics of acoustic materials, and to provide a protective facing material which is easily cleaned and maintained.
The rate at which sound is absorbed in a room or enclosure is a prime factor in reducing noise and controlling reverberation (the persistence of sound due to repeated reflection at the boundaries). Sound is absorbed by a mechanism which converts the sound into other forms of energy and ultimately into heat. The efficiency of a material in absorbing acoustical energy at a specified frequency is given by its absorption coefficient at that frequency. This quantity is the fractional part of the energy of an incident sound wave that is absorbed (not reflected) by the material. For example, if sound waves strike a material in which 55% of the incident acoustical energy is absorbed and 45% is reflected, the sound-absorption coefficient of the material is 0.55.
One important method for determining the sound-absorption coefficient is provided by the reverberation room measurement procedure. With this method, the reverberation time (the measurement of the rate of decay of the sound in an enclosure after the sound source has stopped. Quantitatively, the time in seconds required for the measured sound pressure level in the enclosure to decay by 60 dB) of a room having sound-reflecting walls, floor and ceiling is measured as a function of frequency before and after placing a known area of absorbent in the room. From measurements of reverberation time, taken with and without the absorbent present, the sound-absorption coefficient can be calculated using the well-known Sabine reverberation equation. The absorption coefficient determined in a reverberation room invariably exceeds the true absorption coefficient. One interesting aspect of the problem is the fact that, for a highly-absorbing material, the value of the sound-absorption coefficient can exceed unity, sometimes by as much as 20% or 30%. Of course, it is impossible for a surface to absorb more sound power than its geometrical area would seem to warrent. Since measured results that are greater than the ideal are not yet completely understood, it is recommended in the Standard (ASTM C423-66) that no adjustments be made for this cause. For this reason the sound-absorption coefficient measured from sound decay rates should be denoted and referred to as the Sabine absorption coefficient.
In a full-size reverberation room, the sound absorption due to air predominates at very high frequencies and is strongly dependent upon frequency and relative humidity (R.H.). This fact limits the usable upper frequency range of the measurement technique to approximately 4000 Hz (the upper limit of most standard laboratory measurements). Because present measurement techniques are limited to an upper frequency range of about 4000 Hz, most material manufacturers do not provide absorption data above 4000 Hz. This has made the determination of suitable facing fabrics for high frequency application problematic, particularly for use with airjet looms where frequencies in the 8000-16000 Hz bands predominate. In order to measure the sound-absorption coefficient of a material in the frequency range of 4000-20,000 Hz, it is necessary to scale down the sound absorption of air. This can be accomplished by scaling down the physical size of the reverberation room (i.e., scaling the volume of air absorption) and reducing the relative humidity inside the scale-model room to approximately 1%-2%.
A one-eighth scale-model reverberation room for measuring high-frequency sound-absorption coefficients was designed by scaling down the recommended requirements for a full-scale chamber as given in ASTM C423-66. In practice, the model chamber consisted of a rectangular enclosure constructed of 0.5-inch thick aluminum plate with an interior volume of 13.8 sq. ft. (29".times.31".times.27"). In order to observe the microphone position and gain access to the inside of the chamber, an acrylic sheet roof was used. Model acrylic diffusers were randomly suspended inside the chamber to help provide a diffused sound field. The inside air was dehumidified by circulating dry air from a desiccant compartment to approximately 1% R.H. for the measurements. A physically-small high-frequency loudspeaker was used as a sound source inside the room.
Typically, after the sound source in the enclosure was turned off, the decay of the logarithm of sound pressure (the sound pressure level) versus time was recorded on a digital storage oscilloscope. The reverberation time data was then calculated for each octave-band frequency of interest from 1000-16,000 Hz and entered into the computer for computation of the octave-band sound-absorption coefficient. In order to simplify the analyses of data generated in connection with this invention, a new terminology is introduced High Frequency Noise Reduction Coefficient (HFNRC). This is defined as the arithmetic average of the absorption coefficients in the 4k, 8k and 16k Hz octave bands.
The acoustical absorption characteristics of several potential facing fabrics were obtained in this manner and are discussed more fully below.
Other facing fabric parameters evaluated which have a direct bearing on acoustic properties include material thickness, and air permeability. Parameters more directly related to the fabric cleanability and contamination prevention, such as coefficient of friction, cover factor and abrasion properties were also evaluated. Other relevant considerations include the effects of bonding the facing fabric to the acoustic material, and the effects of coating the facing fabric.
As a result of extensive analysis and testing, it has been determined that a facing fabric possessing good acoustical and cleanability properties should have the following profile:
1. HFNRC: &gt;0.80 (for facing over a 1" thick polyester foam substrate) PA0 2. Fractional Cover Factor: =about 0.8 (no greater) PA0 3. Coefficient of Friction: &lt;about 0.30 PA0 4. Taber Abrasion: &gt;about 400 cycles PA0 5. Air porosity: about 10-50 cfm PA0 6. Fabric Configuration: Plain Weave, continuous filament flat or torque textured yarns with no twist. PA0 7. Facing should be unbonded (or pattern bonded) to acoustical substrate. PA0 8. Facing should not be coated.
In accordance with the above profile, nylon impression fabric (NIF), which receives its name from its conventional use as typewriter ribbon, has been found to combine good acoustical properties with good cleanability and contamination prevention characteristics. Specifically, in one exemplary embodiment of the invention, a 3 or 5 mil thick NIF facing layer is wrapped about a one inch thick foam substrate. It is of some significance that the facing not be bonded to the substrate, although a point or pattern bond system may be acceptable.